Radio frequency transmission cable



Aug. 27, 1935.- H E. ALLEN 2,012,295

RADIO FREQUENCY TRANSMISSION CABLE Filed March 26, 1932 INVENTOR HUGH E. ALLEN ATTOIiNEY Patented Aug. 27, 1935 UNITED STATES PATENT OFFICE RADIO FREQUENCY TRANSMISSION CABLE Hugh E. Allen, Collingswood, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application March 26, 1932, Serial No. 601,412

4 Claims.

I operated with high efliciency from a single antenna without supplying additional power to the transmission line. Due to the low impedance of the transmission line and its perfect shielding, the line and lead-in will not pick up interference. Additionally, in virtue of the uniform, or otherwise suitable, loading and proper termination, reflectionloss is minimized. Thus, radio receiving sets co pled to the transmission line may be tuned independently without audible effects on others connected to the same system. Of course, any other type of load may be placed upon the line at desired points.

An important object of the present invention is to provide a radio frequency distribution system which maintains its calibration over long periods of time and is very stable in operation,

a the system employing a shielded two-conductor transmission cable wherein leakage is a minimum, the cable being not only useful for the distribution of modulated radio frequency energy in a centralized radio system, but being particularly useful for radio frequency distribution in connection with quantitative sensitivity measurements, when it is desired that a known voltage be applied to each of a plurality of receivers, or other loads, under test and connected to the transmission cable.

Still another object of the present invention is to provide in a radio frequency distribution system a low impedance, suitably loaded, metallically shielded transmission cable which is adapted to be coupled to a plurality of radio receivers or other loads.

And still other objects of the invention are to improve generally the efficiency of radio frequency distribution systems, and to particularly provide a low impedance, uniformly] or otherwise suitably, loaded transmission cable which is not only durable and reliable in operation, but economically manufactured and installed.

, The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description" taken in connection with the drawing in which Irhave indicated diagrammatically one circuit arrange-.,

ment whereby my invention may be carried into eflect.

(Cl. 173-266) My present invention relates to an improved,

In the drawing,

Fig.1 is a front view, certain of the parts being broken away, of the transmission cable,

Fig. 2 is an end view of the cable shown in Fig. 1,

Fig. 3 diagrammatically shows a. distribution system utilizing the cable shown in Fig. 1.

Referring to the accomanying drawing wherein like reference characters indicate the same parts in the different figures, in Fig. 1 there is shown a portion of the transmission cable employed in'a radio frequency distribution system. A portion of the cable is shown in Fig. 1, Fig. 2 showing the appearance of the cable looking in the direction along the conductors. Thus, the cable comprises a pair of three-eighth inch fiat copper conductors I, I, the conductors being in sulated from each other by single layer varnish cambric, substantially 0.006 inch thick, the insulating layers being designated by the numerals 2, 2', the width of the insulating layers being, of course, three-eighth inch. The thickness of the copper strips may be .016 inch. Six layers of 0.006 inch varnish cambric are disposed about the layers 2, 2, thus providing a secondary, or outer, insulation layer 3.

Each of the conductors I, I' is preferably wrapped in single layer, pressed together, and then re-wrapped alternately by right and left hand wrap of six layers, all wrapping preferably having three-sixteenths overlap per wrap.

A flat lead sheath 4 of about 0.063 inch thickness, surrounds the outer insulation layer 3, the dimensional relations between the sheath and the remaining elements of the cable being clearly shown in Fig. 2. This type of transmission cable is very well adaptedfor radio frequency distribution because of the fact that the conductors I, I' are balanced with respect to the sheath 4 and ground, and therefore leakage is a minimum. The leakage resistance per foot between the conductors I, I is about 500 megohms, and the voltage break-down between conductors is about 4,000 volts, and 4,000 volts between the lead sheath 4 and the conductors at 60 cycles.

The capacity between one of the conductors and the lead sheath 4 is approximately 0.000150 microfarad per foot at 1,000 kilocycles, while the capacity between conductors is approximately 0.00195 microfarad per foot at 1,000 kilocycles.

The present transmission cable has a low characteristic impedance of the order of 17 ohms for the line, therefore permitting a greater number of receivers on the line as compared with the number of receivers permissible on radio frequency distribution lines employed in present day practice. Lines employed at the present time usually have characteristic impedances of the order of 50 to 65 ohms, and therefore it will be seen that the present transmission cable permits a greater number of receivers on the distribution line in the ratio of at least 50 to 17. It will therefore be seen that the present transmission cable can be employed to great advantage where the requirements for distributing radio frequency energy are severe.

In Fig. 3 there is shown by way of illustration only a radio frequency distribution system utilizing the transmission cable shown in Figs. 1 and 2. Thus, the system comprises an antenna A which is coupled by means of a step-down transformer M to the flat conductors I, l'. The lead sheath 4 is designated by dotted lines, and shown surrounding the conductors I, l'. The end of the transmission line is terminated by a resistor R of proper value and so loads the line as to minimize reflection loss. The antenna coupling transformer M should be so designed as to match the line to the antenna over the band of frequencies it is desired to transmit. By matching the antenna impedance, this transformer permits maximum power to be obtained therefrom, as is well known to those skilled in the art.

At the desired points along the transmission cable outlet, step-up transformer M1 are employed to isolate the receiving sets from the cable, and step-up the signal voltage supplied to them. The conductor I is shown as the ground side of the transmission cable, it being understood that it is only necessary to splice the cable at some desirable point and ground the conductor l' in order to render the conductor l' capable of functioning as the ground side of the system. In that case, of course, the primary and secondary of the transformers M'and M1 would be connected in the manner shown in Fig.

3. It will be noted that the reference letters A1 and G1 designate the antenna and ground terminals of each outlet transformer, so that a receiving set, or other load, may be properly and easily connected to'each outlet transformer.

While only one outlet transformer M1 has been shown, it is to be understood that as many transformers may be employed in the line as there are receivers to be coupled to the line. Those skilled in the art are well aware of the various devices which can be utilized for housing the antenna couplingtransformer M and the outlet transformer M1 it being merely necessary to point out that a junction box may be employed in connection with each transformer, the

transmission cable being run between junction boxes. It is again pointed out that the present transmission .cable not only lends itself in a highly elficient manner to use in connection with the distribution of radio frequency signals to multiple receivers from a single antenna. Due to the fact that it maintains its calibrations very well 'over long periods of time and is very stable in operation, it is particularly useful for radio frequency distribution in connection with quantitative sensitivity measurements, when it is desired that a known voltage be applied to each of the devices under test. a

The absolute values of mechanical and electrical characteristics of the cable which have been given. above are to be considered merely illustrative. The theoretically perfect line would be one having capacitive and inductive reactances which are of. equal: values at the frequency at which the line was to be used. Therefore,

one can see that the lower the characteristic impedance of the line the closer this ideal condition is approached. The particular line described herein has approximately 17 ohms impedance, but if this were of a lower value the line would be still more worth while. By reducing the distance between conductors and increasing the dielectric constant of the separating material, this ideal type of line can be more closely approached.

While I have indicated and described one arrangement for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular shape of cable shown and described, but that many modifications may be made in the curvature of the conductors, sheath and insulation, without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. A transmission cable arranged for theeflicient tors whereby the capacity values between said sheath and each said conductor are equal.

2.. A transmission cable, adapted for the distribution of radio frequency energy, comprising a pair of insulated flat conductors the insulation material separating said conductors being varnished cambric so arranged that the leakage resistance per foot therebetween is of the order of 500 megohms, and a rectangular-shaped lead sheath surrounding said insulated conductors and spaced at equal distances therefrom.

3. A, transmission cable adapted to efficiently transmit a broad band of radio frequency currents from an antenna to a plurality of spaced apart radio receivers coupled thereto said cable comprising only two flat copper strips at least one quarter inch wide, a layer of impregnated insulation material surrounding each strip, the thickness and dielectric constant of said layers being such that said cable has a characteristic impedance of the order of 17 ohms, a plurality of layers of insulation material wrapped around said insulated strips and a rectangular shaped metallic shielding sheath enveloping said copper strips and symmetrically located with reference thereto.

4. A transmission cable adapted to transmit radio frequency currents over a substantial distance, comprising only two fiat thin copper strips each having a width less than one half inch, a layer of insulation material surrounding each .strip of such dielectric material and thickness and each of said strips is substantially the same.

HUGH E. ALLEN. 

